Respiration is one of the most fundamental life processes. It is the process by which living organisms obtain energy from food. While the basic purpose is the same, the way plants and animals respire is quite different. This guide provides a comprehensive overview of the respiratory system in plants and animals, covering key concepts, chemical reactions, enzymes, and a wide range of solved questions to help students excel.
📘 What is Respiration?
Respiration is the biochemical process in which cells break down glucose to release energy, carbon dioxide, and water. This energy is stored in the form of ATP (adenosine triphosphate), which powers all cellular activities.
Respiration is an irreversible chemical change because new substances are formed during the process. It is an exothermic reaction, releasing energy that is essential for life .
🧪 Chemical Reactions of Respiration
Aerobic Respiration (With Oxygen)
This is the most common and efficient form of respiration, occurring in the presence of oxygen.
Word Equation:
Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)
Chemical Equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)
Characteristics:
Releases a large amount of energy (38 ATP molecules per glucose molecule)
End products: Carbon dioxide and water
Anaerobic Respiration (Without Oxygen)
This occurs when oxygen is not available, leading to incomplete breakdown of glucose.
In Animals (including humans during vigorous exercise):
Glucose → Lactic Acid + Energy (2 ATP)
C₆H₁₂O₆ → 2C₃H₆O₃ + Energy (ATP)
In Plants and Yeast:
Glucose → Alcohol + Carbon Dioxide + Energy
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy (ATP)
Characteristics:
Incomplete breakdown of glucose
End products: Lactic acid (in animals) or alcohol and CO₂ (in plants/yeast)
🔬 Enzymes in Respiration
Enzymes are biological catalysts that speed up the chemical reactions of respiration. The respiration process involves about 30 enzyme-controlled steps, and one key enzyme is cytochrome oxidase, which plays a crucial role in the final transfer of electrons to oxygen in aerobic respiration .
Factors affecting enzyme activity in respiration include:
Temperature: Higher temperatures increase reaction rates up to a point, but excessive heat can denature enzymes .
pH: Each enzyme has an optimum pH; deviation can reduce activity .
Substrate and Enzyme Concentration: Higher concentrations can increase the rate of reaction up to a saturation point .
Respiratory Organs in Different Organisms
Different organisms have evolved varied structures for gaseous exchange, depending on their environment and complexity.
🌱 Respiration in Plants
Plants respire all the time—during both day and night. However, during the daytime, the CO₂ produced in respiration is often used up in photosynthesis, so it may not be released to the outside environment. Oxygen is taken in through stomata and lenticels .
Aerobic respiration in plants:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + EnergyAnaerobic respiration in plants (in waterlogged conditions):
Glucose → Alcohol + CO₂ + Energy
🫁 The Human Respiratory System
The human respiratory system is a complex and highly efficient system for gas exchange .
Key Structures:
Nostrils: Air enters through the nostrils.
Pharynx: A tube-like passage common to both the respiratory and digestive systems .
Trachea (Windpipe): A long tube held open by C-shaped cartilage rings, connecting the pharynx to the bronchi .
Lungs: The main organs of respiration, containing millions of alveoli.
Bronchi and Bronchioles: Two narrower tubes that branch from the trachea into each lung, further dividing into smaller bronchioles .
Alveoli: Tiny, balloon-like air sacs at the end of bronchioles. These are the sites of gas exchange .
Diaphragm: A dome-shaped muscle below the lungs that plays a crucial role in breathing .
Mechanism of Breathing:
Inspiration (Inhalation): The diaphragm contracts and flattens, and the ribs move outward. This increases the volume of the chest cavity, decreasing pressure, and causing air to rush into the lungs .
Expiration (Exhalation): The diaphragm relaxes and becomes dome-shaped, and the ribs move inward. This decreases the volume of the chest cavity, increasing pressure, and causing air to be pushed out .
📝 20 Solved MCQs with Answers
1. The process of respiration releases energy in the form of:
(a) Heat only
(b) ATP
(c) Light energy
(d) Kinetic energy
✅ Answer: (b) ATP
2. Which organ does a fish use for respiration?
(a) Trachea
(b) Moist skin
(c) Gills
(d) Spiracles
✅ Answer: (c) Gills – Fish extract dissolved oxygen from water using their gills .
3. The correct chemical equation for aerobic respiration is:
(a) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
(b) C₆H₁₂O₆ → 2C₃H₆O₃ + Energy
(c) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy
(d) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
✅ Answer: (a)
4. Gaseous exchange in plants occurs through:
(a) Spiracles and tracheae
(b) Stomata and lenticels
(c) Gills and skin
(d) Lungs only
✅ Answer: (b) – Plants use stomata on leaves and lenticels on stems for gas exchange .
5. Which of the following is NOT a part of the human respiratory system?
(a) Trachea
(b) Bronchi
(c) Alveoli
(d) Esophagus
✅ Answer: (d) – The esophagus is part of the digestive system .
6. The actual site of gas exchange in the human lungs is:
(a) Trachea
(b) Bronchioles
(c) Alveoli
(d) Nasal cavity
✅ Answer: (c) – Alveoli are tiny air sacs where oxygen and carbon dioxide are exchanged .
7. During anaerobic respiration in animals, glucose is converted into:
(a) Alcohol and Carbon Dioxide
(b) Lactic Acid
(c) Pyruvic Acid
(d) Water and Oxygen
✅ Answer: (b) – In animal cells, anaerobic respiration produces lactic acid .
8. The trachea in humans is supported by:
(a) Bony plates
(b) C-shaped cartilage rings
(c) Muscles
(d) Ligaments
✅ Answer: (b) – C-shaped cartilage rings keep the trachea open .
9. The organ through which an earthworm respires is:
(a) Gills
(b) Lungs
(c) Moist skin
(d) Tracheae
✅ Answer: (c) – Earthworms breathe through their moist skin .
10. Anaerobic respiration in yeast produces:
(a) Lactic acid
(b) Alcohol and Carbon Dioxide
(c) Water and Oxygen
(d) Pyruvic acid
✅ Answer: (b) – Yeast produces alcohol and carbon dioxide during anaerobic respiration (fermentation) .
11. The function of the epiglottis is to:
(a) Produce sound
(b) Prevent food from entering the trachea
(c) Warm the air
(d) Filter dust
✅ Answer: (b) – The epiglottis closes over the trachea during swallowing .
12. Respiration in insects like cockroaches occurs through:
(a) Gills
(b) Lungs
(c) Tracheal system
(d) Moist skin
✅ Answer: (c) – Insects use a tracheal system with spiracles and tracheae .
13. During inspiration, the diaphragm:
(a) Flattens
(b) Becomes dome-shaped
(c) Relaxes
(d) Moves upward
✅ Answer: (a) – The diaphragm contracts and flattens during inspiration .
14. The primary function of alveoli is to:
(a) Produce oxygen
(b) Exchange gases
(c) Filter blood
(d) Absorb water
✅ Answer: (b) – The alveoli are responsible for gas exchange .
15. Oxygen is transported in human blood as:
(a) Oxyhaemoglobin
(b) Carboxyhaemoglobin
(c) Carbaminohaemoglobin
(d) Deoxyhaemoglobin
✅ Answer: (a) – Oxygen binds with haemoglobin to form oxyhaemoglobin .
16. The correct sequence of air passage in humans is:
(a) Nostrils → Larynx → Pharynx → Trachea → Bronchi → Lungs
(b) Nostrils → Pharynx → Larynx → Trachea → Bronchi → Lungs
(c) Nostrils → Pharynx → Trachea → Larynx → Bronchi → Lungs
(d) Nostrils → Larynx → Trachea → Pharynx → Bronchi → Lungs
✅ Answer: (b)
17. Which of the following is NOT part of the conducting zone of the respiratory system?
(a) Pharynx
(b) Nasal cavity
(c) Alveoli
(d) Bronchi
✅ Answer: (c) – The conducting zone includes structures that move air, while the respiratory zone (alveoli) is where gas exchange occurs .
18. The maximum number of ATP molecules produced from one glucose molecule during aerobic respiration is:
(a) 2
(b) 12
(c) 36
(d) 38
✅ Answer: (d) – Aerobic respiration produces 38 ATP molecules per glucose molecule .
19. In plants, CO₂ generated during respiration in the daytime is:
(a) Released into the atmosphere
(b) Used up in photosynthesis
(c) Converted into oxygen
(d) Stored in the roots
✅ Answer: (b) – During daytime, CO₂ from respiration is used in photosynthesis .
20. Which of the following is a characteristic of alveoli that helps in gas exchange?
(a) They are thick-walled
(b) They have a large surface area
(c) They are not supplied with blood
(d) They are filled with mucus
✅ Answer: (b) – Alveoli have thin walls and a large surface area, surrounded by blood capillaries for efficient gas exchange .
📝 15 Solved Short Answer Questions (2-3 Marks)
Q1. What is the difference between respiration and breathing?
Answer: Breathing is the physical process of inhaling and exhaling air (ventilation), while respiration is the biochemical process of breaking down glucose to release energy in cells .
Q2. Name the respiratory organs of fish and earthworm.
Answer: Fish use gills for respiration, and the earthworm uses its moist skin (cuticle) .
Q3. What is the role of the diaphragm in breathing?
Answer: The diaphragm is a dome-shaped muscle below the lungs. Its contraction and relaxation change the volume of the chest cavity, causing air to be drawn in (inhalation) or expelled (exhalation) .
Q4. Define aerobic respiration and give its equation.
Answer: Aerobic respiration is the process of breaking down glucose in the presence of oxygen to release energy. Equation: Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP) .
Q5. Where does gaseous exchange occur in plants?
Answer: Gaseous exchange in plants occurs through stomata (on leaves) and lenticels (on stems) .
Q6. What is the end product of anaerobic respiration in human muscle cells?
Answer: The end product of anaerobic respiration in human muscle cells is lactic acid .
Q7. Why is the skin of an earthworm always kept moist?
Answer: An earthworm’s skin is kept moist to allow oxygen to dissolve in the moisture, facilitating its diffusion into the body. The moist skin is its primary respiratory organ .
Q8. What is the function of ciliated epithelium in the respiratory tract?
Answer: The ciliated epithelium lines the respiratory passage and helps to remove dust particles and mucus by moving them upward towards the throat .
Q9. Which enzyme is crucial for the final step of aerobic respiration?
Answer: Cytochrome oxidase is a key enzyme involved in the final transfer of electrons to oxygen during aerobic respiration .
Q10. Why do fish have gills?
Answer: Fish have gills, which are rich in blood vessels, to extract dissolved oxygen from water. Water passes over the gills, and oxygen diffuses into the blood .
Q11. What is the main organ of respiration in birds?
Answer: Lungs are the main organs of respiration in birds .
Q12. Why is anaerobic respiration less efficient than aerobic respiration?
Answer: Anaerobic respiration produces much less energy per glucose molecule (2 ATP) compared to aerobic respiration (38 ATP) because the glucose molecule is not completely broken down .
Q13. What is the role of the trachea?
Answer: The trachea, or windpipe, is a tube that connects the pharynx to the bronchi, providing a passageway for air to enter the lungs .
Q14. Why is the surface area of the alveoli important?
Answer: The large surface area of alveoli allows for a very efficient and rapid exchange of oxygen and carbon dioxide between the air in the lungs and the blood in surrounding capillaries .
Q15. What are spiracles?
Answer: Spiracles are tiny openings on the abdomen of insects through which air enters their tracheal system for respiration .
📝 12 Solved Long Answer Questions (5 Marks)
Q1. Explain the mechanism of breathing in humans with the help of a diagram.
Answer: Breathing involves two phases:
Inspiration (Inhalation): The diaphragm contracts and flattens, moving downward. The intercostal muscles contract, pulling the ribs upward and outward. This increases the volume of the thoracic (chest) cavity, decreasing the pressure inside the lungs. Air rushes in from the outside through the nostrils, pharynx, larynx, trachea, bronchi, and bronchioles to fill the alveoli .
Expiration (Exhalation): The diaphragm relaxes and becomes dome-shaped, moving upward. The intercostal muscles relax, allowing the ribs to move downward and inward. This decreases the volume of the thoracic cavity, increasing the pressure inside the lungs. Air is forced out of the lungs through the same passage .
Q2. Describe the process of gaseous exchange in the alveoli.
Answer: The alveoli are the primary sites of gas exchange in the lungs. They are thin-walled air sacs surrounded by a network of blood capillaries. The walls of the alveoli and the capillaries are very thin (one cell thick). During inhalation, oxygen-rich air fills the alveoli. Oxygen, being at a higher concentration in the alveoli than in the blood, diffuses across the thin alveolar and capillary walls into the blood. This oxygen binds to haemoglobin in red blood cells to form oxyhaemoglobin for transport. Conversely, carbon dioxide, which is at a higher concentration in the blood, diffuses from the blood into the alveoli to be exhaled .
Q3. Compare and contrast aerobic and anaerobic respiration.
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen Requirement | Requires oxygen | Does not require oxygen |
| Glucose Breakdown | Complete breakdown | Incomplete breakdown |
| Energy Yield | High (38 ATP) | Low (2 ATP) |
| End Products | CO₂ and Water | Lactic acid (in animals) or Alcohol + CO₂ (in yeast) |
| Efficiency | Highly efficient | Less efficient |
| Organisms | Most organisms (humans, plants, animals) | Some bacteria, yeast, and muscle cells under stress |
Q4. Write the respiratory organs of the following animals and explain how they work: (a) Fish (b) Insects (c) Earthworm.
(a) Fish: Fish use gills to breathe. They take in water through their mouth, which passes over the gill filaments. These filaments are richly supplied with blood capillaries. Oxygen dissolved in the water diffuses into the blood, and carbon dioxide diffuses out into the water, which is then expelled through the gill openings.
(b) Insects: Insects use a tracheal system. Air enters the body through tiny openings called spiracles on the abdomen. From the spiracles, air travels through a network of tubes called tracheae and tracheoles. These tubes deliver oxygen directly to the tissues and cells of the insect’s body.
(c) Earthworm: An earthworm breathes through its moist skin (cuticle). Oxygen dissolves in the layer of moisture on the skin and then diffuses into the blood capillaries underneath. Carbon dioxide diffuses in the opposite direction. The skin must remain moist for this diffusion to occur.
Q5. Describe the process of respiration in plants.
Answer: Plants respire all the time (both day and night) to get energy for their life processes. Unlike animals, plants do not have specialized respiratory organs. Gaseous exchange occurs through simple diffusion.
Leaves: The main site of gas exchange in leaves is through tiny pores called stomata. Oxygen enters the leaf through stomata for respiration, and carbon dioxide produced diffuses out.
Stems and Roots: In woody stems, gas exchange occurs through lenticels (small openings in the bark). Roots also respire, taking in oxygen present in the soil air.
The equation for respiration in plants is the same as in animals: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy. However, during the daytime, the CO₂ produced in respiration can be used directly for photosynthesis .
Q6. What are the adaptations of the human respiratory system for efficient gas exchange?
Answer: The human respiratory system has several key adaptations to ensure efficient gas exchange :
Large Surface Area: The millions of alveoli in the lungs provide an enormous surface area for gas exchange.
Thin Membranes: The walls of the alveoli and the capillaries are extremely thin (single cell thick), providing a short diffusion distance for gases.
Rich Blood Supply: The alveoli are surrounded by a dense network of blood capillaries, ensuring a constant flow of blood to pick up oxygen and release carbon dioxide.
Moist Surface: The inner lining of the alveoli is moist, allowing gases to dissolve before diffusing across the membrane.
Ventilation: The breathing mechanism (using the diaphragm and ribs) constantly refreshes the air in the lungs, maintaining a high concentration gradient for oxygen and a low concentration gradient for carbon dioxide.
Q7. Explain the role of enzymes in the process of respiration.
Answer: Respiration is a complex metabolic pathway involving a series of many chemical reactions. Enzymes are crucial because they are biological catalysts that speed up these reactions, making respiration fast enough to sustain life .
Specificity: Each step in the respiration process is catalyzed by a specific enzyme. For example, enzymes like proteases and peptidases are involved in breaking down proteins to amino acids before they can enter the respiratory pathway .
Regulation: Enzymes control the rate of respiration. Factors like temperature and pH can affect enzyme activity, influencing the overall rate of cellular respiration .
Cytochrome Oxidase: This is a specific enzyme in the electron transport chain that is essential for the final step of aerobic respiration, where oxygen is reduced to water .
In short, without enzymes, the chemical reactions of respiration would be too slow to provide the energy needed for survival.
Q8. Differentiate between breathing and respiration.
| Breathing (Ventilation) | Respiration (Cellular) |
|---|---|
| It is the physical process of moving air or water into and out of the lungs or other respiratory organs. | It is the biochemical process of breaking down glucose to release energy in cells. |
| It involves organs like the nose, trachea, and lungs. | It occurs in the mitochondria and cytoplasm of cells. |
| It does not produce energy. | It produces energy in the form of ATP. |
| It is an extracellular process. | It is an intracellular process. |
| It is a mechanical process. | It is a chemical process. |
Q9. Describe the process of anaerobic respiration in yeast and its importance.
Answer: Anaerobic respiration in yeast is also known as fermentation. When oxygen is not available, yeast cells break down glucose to produce alcohol (ethanol), carbon dioxide, and a small amount of energy .
Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy (ATP)
Importance:
Alcohol Production: This process is the basis for the production of alcoholic beverages like wine and beer.
Baking Industry: The carbon dioxide produced during fermentation is used in the baking industry to make bread and cakes rise. The CO₂ gas gets trapped in the dough, causing it to expand.
Q10. Why do frogs have three respiratory organs? How does it help them?
Answer: Frogs are amphibians, meaning they can live both on land and in water. This dual lifestyle has led to the evolution of three respiratory organs: lungs, skin, and the lining of the mouth (buccal cavity) .
How it helps:
On Land: They primarily use their lungs (pulmonary respiration) to breathe atmospheric air.
In Water: They use their moist skin (cutaneous respiration) to absorb oxygen dissolved in water.
At Rest: They can also use their buccal cavity (mouth lining) for gas exchange.
Having multiple respiratory organs allows frogs to adapt their breathing method based on their environment and activity level. Cutaneous respiration is particularly important when they are in water.
Q11. Explain the process of inhalation and exhalation in humans.
Inhalation (Breathing in):
The diaphragm contracts and moves downward (flattens).
The intercostal muscles (between the ribs) contract, pulling the ribs up and out.
These actions increase the volume of the chest cavity.
As the volume of the chest cavity increases, the pressure inside the lungs decreases below the atmospheric pressure.
This creates a pressure difference, and air is drawn into the lungs through the nose or mouth.
Exhalation (Breathing out):
The diaphragm relaxes and moves upward (becomes dome-shaped).
The intercostal muscles relax, and the ribs move down and in.
These actions decrease the volume of the chest cavity.
As the volume of the chest cavity decreases, the pressure inside the lungs increases above the atmospheric pressure.
This forces air out of the lungs.
Q12. How are respiratory diseases like asthma and bronchitis caused? What are their symptoms?
Asthma:
Causes: Asthma is often caused by allergic reactions to substances like pollen, dust, or smoke. It can also be triggered by exercise or stress.
Symptoms: The main symptoms include wheezing (a whistling sound while breathing), coughing, shortness of breath, and a feeling of tightness in the chest. It occurs because the airways become inflamed and narrowed.
Bronchitis:
Causes: Bronchitis is commonly caused by a viral or bacterial infection. It can also be triggered by smoking or exposure to irritants.
Symptoms: Symptoms include a persistent cough with mucus production, chest discomfort, and difficulty breathing. It is an inflammation of the bronchial tubes (bronchi).
🌟 How Dhingra Classes Helps Students Master Biology
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